One distinction which I find important is that HTML::Template is for HTML, whereas TT2 is a general-purpose templating system. I've used the latter for HTML/XHTML, XML, CSS, LaTeX and an assorted bunch of weird and wonderful proprietary data formats. Given that TT2 does all this, I haven't felt any great need to learn HTML::Template's syntax as well.

If all you will ever want to produce are HTML templates then this probably won't matter to you, but my preference is for the more flexible tool.